1. Field of the Invention
The present invention relates to a profiled sheathed resistive wire which can be attached in various ways to an adjacent like wire. This wire is particularly intended for forming a heating insert for electric resistance welding plastics material pipes, such as gas distribution pipes which are usually made from high-density polyethylene.
2. Description of the Prior Art
The electric resistance welding technique consists in using a heating wire embedded in a connecting part and forming an electrical resistance to heat the contacting surfaces of the parts to the temperature at which the materials weld.
The usual technique employed for forming such connecting parts (or connectors) is overmolding by injection molding. The heating wire is placed in a spool which is overmolded to form the connecting part. The wire is therefore relatively near the inside surface of the element. To achieve good quality welding it is desirable for the wire to be as near as possible to this inside contact surface.
The fabrication of this sleeve, to be subsequently overmolded, is therefore a major process in the manufacture of electric resistance welding connectors.
As a general rule the wire is wound directly onto the relatively thin sleeve and fixed by heating the wire, for example (GLYNWED TUBES AND FITTINGS LIMITED European patent 0303 909), or inserted into a helical groove preformed on the sleeve (WAVIN European patent 0262 735).
It has also been proposed to form prefabricated inserts (or preforms) from sheathed wire.
This is the case with the electric resistance welding connector of European patent 0036 963 (GEORG FISHER AKTIEN GESELLSCHAFT) in which a cylindrical coated wire is wound into a helix and the coating of each turn is solidly welded to the coating of the next turn during the fabrication of the winding, to form a compact bush.
It is also an insert of this type which is described in French patent 2 503 020 (SERAP), in which a sheathed wire is wound with contiguous turns onto an expandable mandrel, the wire is heated sufficiently for the sheath to soften and the mandrel is expanded by a conical core so that the outside surface of the mandrel is practically in contact with a generatrix of the resistive wire. In this way the turns are welded together to produce a sleeve intended to be inserted into an electric resistance welding connector.
In another technique disclosed in French patent 2 040 143 (GERBERT et Cie), the heating conductor is a strip of plastics material on which the heating wire is disposed in separated turns. This heating conductor is then introduced between the tubes to be jointed.
Heating inserts are also used in connecting parts employed to make a branch connection by forming a hole in situ in a pipe.
Connecting parts of this kind are usually in two parts receiving the pipe. The hole is formed after heat welding the pipe in the area in which the hole is to be made, by means of an electrical heating element.
In a similar way to the case of the connecting sleeve, attempts have been made to produce a prefabricated heating insert.
In the prior art described in French patent 2 171 223 (ROLLMAPLAST AG), for example, a sheathed resistive wire of spiral or other form is overmolded into the mass of the saddle part.
A prefabricated insert is described in French patent 2 519 578 (INNOVATION TECHNIQUE SAM); this insert is formed from a relatively thin molded plate incorporating a groove into which the spiral-form heating wire is placed; this insert is designed to be incorporated into the inside surface of the saddle, preferably during molding of the latter.
Prefabrication of these heating inserts allows large-scale series manufacture, guarantees high quality connecting parts and significantly facilitates the molding or overmolding process.
To summarize, this prefabrication is primarily achieved by first (before the connecting part itself is fabricated) connecting the heating wire to a plastics material base of appropriate shape or by welding the sheath of the wire direct to that of the adjacent wire by heating on an appropriate supporting surface.
These techniques produce parts of inserts specific to each application, that is to say sleeves in the case of conventional cylindrical joints and disks or plates in the case of perforation joints, and also specific to each diameter of the tubular members to be jointed.
The object of the invention is to combine the advantage of prefabrication with that of a multifunction wire that can be used for multiple applications.